From the documentation:

It is generally recommended to omit the public modifier for test classes, test methods, and lifecycle methods unless there is a technical reason for doing so – for example, when a test class is extended by a test class in another package. Another technical reason for making classes and methods public is to simplify testing on the module path when using the Java Module System.

(Emphasis mine.)

I remember that in the past an opens my.pkg; directive was required in the module-info.java file was required to execute package-private test classes/methods residing in that package.

Recently I removed such an existing opens and expected some package-private tests to fail – but they ran successfully.

With a recent JUnit version, what does "simplify testing on the module path" mean?